The ROSAT's mission goals are two-fold: First, using the scan mode,
a complete all-sky survey
is to be carried out with imaging telescopes in order to detect
sources at X-ray and XUV energies,
measure their positions with an accuracy of less than 0.5',
and obtain fluxes and broad-band spectra.
Second, using the pointed mode,
selected sources will be studied in detail with respect to
spatial extent, temporal variability and spectral properties.